Kids need time outside. It seems basic, but we're still re-learning that lesson after totally forgetting it for about a generation. Two new studies reinforce the importance of the outdoors for kids' health.

One, from Dr. Frances E. Kuo of the Landscape and Human Health Laboratory at the University of Illinois, showed that a twenty-minute walk in a natural urban park improved the concentration ability of children diagnosed with Attention-Deficit Hyperactivity Disorder as much as or more than medication.

"What this particular study tells us is that the physical environment matters," says Dr. Kuo in "The Well" blog in the New York Times. Although Kuo and her collaborators have shown in other studies that nature affects learning and concentration, this is the first research to show measurable improvements in children with ADHD due to time spent in green spaces.

That's exciting news for parents, because "doses of nature" could serve as a "safe, inexpensive, widely accessible new tool in the tool kit" for managing ADHD symptoms, write Kuo and her co-author, Andrea Faber Taylor in the Journal of Attention Disorder.

The other study, a report from the American Academy of Pediatrics in Boston recommends doubling the minimum daily requirement for Vitamin D for children, from 200 IU to 400. What does that have to do with children and the out-of-doors? Our skin naturally synthesizes Vitamin D on exposure to sunlight. So while the ultraviolet rays in sunlight can be damaging, we need sun exposure to ensure healthy Vitamin D levels. And not just sunlight coming through a window, since glass blocks most of the rays needed to synthesize Vitamin D.

The American Academy of Pediatrics notes that Vitamin D is added to foods, especially milk and other dairy products, but kids in general don't eat enough of those foods to gain the recommended levels. Vitamin D, a hormone that regulates gene transcription, the copies of genes made when cells divide, is critical in preventing rickets (brittle bones), as well as in preventing respiratory infections in infants, growth failure, lethargy, and osteoporosis. A deficiency of this hormone made using sunlight has recently been tied to type 2 diabetes, some types of cancer, multiple sclerosis, and rheumatoid arthritis. So the organization of kids' doctors is recommending—vitamin supplements. Not time outside.

Maybe we're still re-learning the lesson: Kids need time outdoors. For their health, and their future well-being.